(l-tē'shē-əm) Symbol LuA silvery-white metallic element of the lanthanide series that is used in nuclear technology. Its radioactive isotope is used in determining the age of meteorites. Atomic number 71; atomic weight 174.97; melting point 1,663°C; boiling point 3,395°C; specific gravity 9.840 (at 25°C); valence 3. See Periodic Table.

(Lu), chemical element, rare-earth metal of the lanthanoid series of the periodic table; the hardest and densest rare-earth element, last member of the lanthanoid series. Lutetium was discovered (1907-08) by Carl Auer von Welsbach and Georges Urbain, working independently. Urbain derived the name for the element from Lutetia, which was the ancient Roman name for Paris. The name Urbain gave it to honour Paris, his native city, became widely accepted, except in Germany, where it was commonly called cassiopeium until the 1950s. One of the rarest of the rare earths, lutetium occurs in rare-earth minerals such as xenotime and euxenite. Though it composes only about 0.003 percent of the commercially important mineral monazite, it has proved feasible to extract it as a by-product. Separation and purification are accomplished by ion-exchange techniques. Lutetium is also found in the products of nuclear fission. The metal has been prepared by thermoreduction of the anhydrous halides by alkali or alkaline-earth metals. It has the highest melting point of the rare-earth elements. Natural lutetium consists of two isotopes: stable lutetium-175 (97.41 percent) and radioactive lutetium-176 (2.59 percent, 3 1010-year half-life). The radioactive isotope is used to determine the age of meteorites relative to that of the Earth. Few other uses have been found for lutetium.